Communications equipment, computers, home stereo amplifiers, televisions, and other electronic devices are increasingly manufactured using small electronic components which are very vulnerable to damage from electrical energy surges (i.e., transient over-voltages). Surge variations in power and transmission line voltages can severely damage and/or destroy electronic devices. Moreover, these electronic devices can be very expensive to repair and replace. Therefore, a cost effective way to protect these components from power surges is needed. Devices such as zener diodes and thyristors have been developed to protect these types of equipment from such power surges or over-voltage transients. These devices, typically discrete devices similar to discrete voltage-reference diodes, are employed to suppress transients of high voltage in a power supply or the like before the transients reach and potentially damage an integrated circuit or similar structure. A simple pn junction does not generally meet the requirements for reverse breakdown operation since, in practice, reverse breakdown occurs at a relatively unpredictable voltage where the junction meets a surface of the p and n regions.
FIG. 1 shows a two layer semiconductor junction diode of the type disclosed in UK Patent Appl. 2113907A, which provides a more predictable reverse breakdown voltage than is achievable with a simple pn junction. The junction diode includes an n-type substrate and a diffused p-type region formed in the n-type substrate. The diode has a conventional planar junction which has a selectively flat central region and curved edge regions which terminate at the surface of the n-type substrate. A buried n-type region 3 lies in the n-type substrate 1, adjacent to the central region of the planar junction. The p-type region 2 has a higher impurity concentration than both the n-type substrate 1 and the buried n-type region 3, while the buried n-type region 3 has a higher impurity concentration than the substrate 1.
FIG. 2 represents the impurity concentration profile of the pn junction of FIG. 1. As shown, the p-type region 2 extends from the surface of the diode to a depth of about 20 microns and varies in impurity concentration from about 1019 atoms/cc to that of n-type substrate 1, which is about 1015 atoms/cc. The buried n-type region 3 extends from the surface of the diode to a depth of about 40 microns and varies in impurity concentration from about 1017 atoms/cc to that of the n-type substrate 1. The p-type region 2, being initially of higher impurity concentration than the buried n-type region 3, is the dominant region to a depth of about 15 microns. The buried n-type region 3 dominates in the depth range from about 15 to 40 microns.
The presence of the buried n-type region 3 causes a reduction in the reverse breakdown voltage for the junction in the bulk material. The buried n-type region 3 modifies the junction structure so that under a reverse bias breakdown occurs through the buried region 3. The aforementioned patent application also shows a 4 layer diode in which a p-type anode region contacts the bottom surface of the substrate 1 and an n-type cathode region contact the p-type region 2. A similar 4 layer diode that employs a series of buried regions similar to buried n-type region 3 of FIG. 1 is shown in U.S. Pat. No. 5,516,705. The 4 layer diode shown in this patent also includes shorting dots in the n-type cathode region. The shorting dots are regions contiguous with the upper metal contact and are employed to improve the accuracy of the gating characteristics as a function of temperature.
While the use of the buried n-type region 3 shown in FIG. 1 of the conventional semiconductor junction diode device provides a more predictable reverse breakdown voltage, it also causes a reduction in the reverse breakdown voltage. For many applications however, a large value for the reverse breakdown voltage is desired.